T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological malignancy. Despite improved patient outcomes with dose intensification chemotherapy, 20% of pediatric and 50% of adult patients do relapse and ultimately die because of refractory disease. Therefore, there is a critical need for novel, highly effective anti-leukemia therapeutics. This project aims to establish the mechanisms of action of FBXL4 (F-box and leucine-rich repeat protein 4), a gene frequently inactivated in T-ALL harboring deletions in the long arm of chromosome 6 (6q-). Although 6q deletions are often large and heterogeneous, we have identified a focal 6q deletion encompassing just the FBXL4 gene. Interestingly, FBXL4 is differentially expressed during thymocyte differentiation, suggesting a functional role for FBXL4 in early T-cell development. The FBXL4 gene encodes an F-box and leucine-rich repeat factor with a predicted role in targeting specific proteins for proteasomal degradation. F-box proteins participate in essential cellular processes, including cell cycle progression, response to DNA damage, and survival. Moreover, dysregulation of F-box proteins such as FBXW7 and FBXO11 contributes to cellular transformation. At this point, the substrates and the molecular and cellular functions of the F-box protein FBXL4 remain largely unknown. To elucidate the roles and mechanisms of FBXL4 in T-ALL, I will: (1) investigate the functional consequences of FBXL4 loss in T-ALL in cell proliferation, survival, differentiation, and response to chemotherapy; (2) identify the substrates targeted for proteasomal degradation by FBXL4 in T-ALL cells; and (3) characterize the role of FBXL4 in T-cell development and transformation in vivo using conditional Fbxl4 knockout mice. Overall, these studies will characterize the roles and mechanisms of FBXL4 in the pathogenesis of T-ALL.